1. Field of the Invention
The present invention relates to a transflective liquid crystal display device, an electronic device comprising the same, and a controller for a transflective liquid crystal display device.
2. Description of the Related Art
A transflective liquid crystal display device is one type of liquid crystal display device, and some transflective liquid crystal display devices are capable of switching between a display mode for displaying an image on a screen and a mirror mode for placing the screen into a mirror state. Such a liquid crystal display excels not only in practicability but also in decorativeness.
Also, liquid crystal display devices may conform to several display modes such as TN (Twisted Nematic) scheme, ECB (Electrically Controlled Birefringence) scheme, VA (Vertical Alignment) scheme, IPS (in Plane Switching) scheme, and the like.
JP2004-170792A describes a TN-based transflective liquid crystal display device and an ECB-based transflective liquid crystal display device.
The TN-based liquid crystal display device described in JP2004-170792A will be described with reference to FIGS. 1 and 2. FIGS. 1 and 2 are cross-sectional views generally showing the configuration of the liquid crystal display device in its thickness direction.
Referring first to FIG. 1, a description will be given of the configuration of the liquid crystal display device. This liquid crystal display device comprises liquid crystal panel 920 for displaying an image, and back light 970 which is a light source for irradiating light onto a bottom surface of liquid crystal panel 920. With this liquid crystal display device, a user can observe liquid crystal panel 920 as a screen from above liquid crystal panel 920.
Liquid crystal panel 920 comprises upper substrate 930 and lower substrate 950 which are arranged in opposition to each other. Upper substrate 930 is provided with polarizer plate 910 on its top surface, while lower substrate 950 is provided with polarizer plate 960 on its bottom surface.
Coloring layer 941a covered with protection film 941b is disposed on the bottom surface of upper substrate 930, and common electrode 942 is disposed on a bottom surface of protection film 941b. On the top surface of lower substrate 950, in turn, reflector plate 945 is disposed, where openings 949 are sequentially formed side by side through reflector plate 945. Electrodes 944 are disposed on the top surface of reflector plate 945 and in openings 949.
Liquid crystal layer 943 filled with liquid crystal is interposed between upper substrate 930 and lower substrate 950. When no voltage is applied between common electrode 942 and electrode 944, liquid crystal layer 943 is oriented in twisted alignment where liquid crystal molecules sequentially twist by 90 degrees between substrates 930 and 950, causing the direction of linearly polarized light, which is transmitted through liquid crystal layer 943, to rotate by 90 degrees. On the other hand, when a sufficient voltage is applied between common electrode 942 and electrode 944, liquid crystal layer 943 is such that liquid crystal molecules are aligned vertically with respect to substrates 930, 950, causing no change in the polarization state of the linearly polarized light which is transmitted through liquid crystal layer 943. Here, a “non-voltage applied state” refers to a state where no voltage is applied between common electrode 942 and electrode 944, while a “voltage applied state” refers to a state where a sufficient voltage is applied between common electrode 942 and electrode 944.
Coloring layer 941a is disposed at a position opposite to opening 949. Coloring layer 941a is a layer which colors light irradiated from back light 970 in one of red (R), green (G), and blue (B) by allowing the light to be transmitted through coloring layer 941a upward from blow.
Accordingly, as light irradiated from back light 970 passes through opening 949 in the display mode, the light is transmitted through coloring layer 941a and is thereby colored. In this way, this liquid crystal display device can display a color image on the screen because it can emit colored light upward through liquid crystal panel 920.
In the mirror mode, on the other hand, external light incident on the liquid crystal display device from above polarizer plate 910 is reflected by reflector plate 945, and the reflected light is emitted upward from polarizer plate 910. In this way, liquid crystal panel 920 appears like a mirror, as viewed from above, in the mirror mode. In this regard, since the external light incident on polarizer plate 910 is not transmitted through coloring layer 941a in a process where it is reflected by reflector plate 945 and emitted from polarizer plate 910, the reflected light is emitted without being colored.
Referring next to FIG. 2, a description will be given of the operation of the TN-based liquid crystal display device. Polarizer plate 910 and polarizer plate 960 are disposed such that their polarization transmission axes are orthogonal to each other. Specifically, polarizer plate 910 exhibits a polarization transmission axis in a direction parallel to the drawing sheet of FIG. 2 as indicated by circled arrows in FIG. 2, while polarizer plate 960 exhibits a polarization transmission axis in a direction perpendicular to the drawing sheet as indicated by a circled mark “X.”
In the non-voltage applied state of this liquid crystal display device, arrow 801 indicates a trajectory of light irradiated from back light 970, and arrow 802 indicates a trajectory of external light which incident on polarizer plate 910 from above. As indicated by the arrows, polarizer plate 910 is transmitted by the light irradiated from back light 970, and is also transmitted by the external light which is incident on polarizer plate 910 from above and reflected by reflector plate 945.
In the voltage applied state of this liquid crystal display device, on the other hand, arrow 804 indicates a trajectory of light irradiated from back light 970, and arrow 803 indicates a trajectory of external light incident on polarizer plate 910 from above. As indicated by these arrows, the light emitted from back light 970 is not transmitted through polarizer plate 910 but is absorbed by polarizer plate 910, while the external light incident on polarizer plate 910 from above and reflected by reflector plate 945 is transmitted through polarizer plate 910.
In this liquid crystal display device, since the light irradiated from back light 970 is allowed to be transmitted through polarizer plate 910 upward by placing the device into the non-voltage applied state, the liquid crystal display device can be set to the display mode where an image can be displayed on the screen. On the other hand, in this liquid crystal display device, since the external light reflected by reflector plate 945 is allowed to be transmitted through polarizer plate 910 upwards, while the light irradiated from back light 970 is not allowed to be transmitted through polarizer plate 910 upwards, by placing the device into the voltage applied state, the liquid crystal display device can be set to the mirror mode where the screen can be used as a mirror.
Referring next to FIGS. 3 and 4, a description will be given of an ECB-based liquid crystal display device described in JP2004-170792A. FIGS. 3 and 4 are schematic diagrams showing the configuration of this liquid crystal display device.
Referring first to FIG. 3, a description will be given of the configuration of the liquid crystal display device. This liquid crystal display device is constructed in a similar manner to the TN-based liquid crystal display device shown in FIGS. 1 and 2 except that liquid crystal panel 920a is provided with first λ/4 plate 918, second λ/4 plate 919, and insulating layer 990, and that liquid crystal molecules are oriented in twisted alignment where they sequentially twist between substrates 930 and 950 by a value which is set in a range of zero to 90 degrees. In FIGS. 3 and 4, components common to FIGS. 1 and 2 are designated the same reference numerals.
λ/4 plate 918 is disposed between upper substrate 930 and polarizer plate 910, while λ/4 plate 919 is disposed between lower substrate 950 and polarizer plate 960. Also, insulating layer 990 is disposed between lower substrate 950 and reflector plate 945 in order to position a reflecting surface of reflector plate 949 at the center of liquid crystal layer 943 in a thickness direction. λ/4 plate 918 and λ/4 plate 919 are wavelength plates for transforming linearly polarized light into circularly polarized light and transforming circularly polarized light into linearly polarized light.
Referring next to FIG. 4, a description will be given of the operation of this ECB-based liquid crystal display device.
In the non-voltage applied state of the liquid crystal display device, arrow 805 indicates a trajectory of light irradiated from back light 970, while arrow 806 indicates a trajectory of external light incident on polarizer plate 910 from above. In this way, polarizer plate 910 is transmitted by the light irradiated from back light 970, and is also transmitted by the external light which is incident on polarizer plate 910 from above and reflected by reflector plate 945.
In the voltage applied state of the liquid crystal display device, arrow 808 indicates a trajectory of light irradiated from back light 970, while arrow 807 indicates a trajectory of external light incident on polarizer plate 910 from above. In this way, the light irradiated from back light 970 is not transmitted through polarizer plate 910 but is absorbed by polarizer plate 910, and the external light incident on polarizer plate 910 from above and reflected by reflector plate 945 is not transmitted through polarizer plate 910 but is absorbed by polarizer plate 910.
In this liquid crystal display device, since the light irradiated from back light 970 is allowed to be transmitted through liquid crystal panel 920a upward by placing the device into the non-voltage applied state, the liquid crystal display device can be set to the display mode where an image can be displayed on the screen. On the other hand, in this liquid crystal display device, since the external light reflected by reflector plate 945 alone is allowed to be transmitted through liquid crystal panel 920a upward by placing the device into the non-voltage applied state, and turning off back light 805, the liquid crystal display device can be set to the mirror state where the screen can be used as a mirror.
In the TN-based liquid crystal display device shown in FIGS. 1 and 2, when a black character is displayed on a white background, for example, in the display mode, the voltage applied state is set in those pixels which display the black character, to prevent the light irradiated from back light 970 from being transmitted through liquid crystal panel 920 upwards. However, in the voltage applied state, reflected light reflected by reflector plate 945 is also emitted through liquid crystal panel 920 upward. Therefore, in a bright place such as outdoors on a clear day, the pixels which display the black character are observed to be bright by the user due to the reflected light from reflector plate 945, so that the contrast of the black character appears to be lower with respect to the white background. For this reason, this liquid crystal display suffers from lower visibility in bright places.
The ECB-based liquid crystal display device shown in FIGS. 3 and 4 is free from lower visibility as described above because neither irradiated light from back light 970 nor reflected light from reflector plate 945 are allowed to exit through liquid crystal panel 920a upwards in the voltage applied state. However, in this liquid crystal display device, reflected light from reflector plate 945 is also allowed to exit through liquid crystal panel 920a upwards in the display mode, so that the reflected light, not colored, mixes with colored irradiated light from back light 970 when a color image is displayed. Consequently, this liquid crystal display device suffers from lower saturation when an image is displayed in color.